Raman spectroscopy, characteristics of Raman spectrum, origin of stokes and antistokes line in Raman spectrum,

1. Dr. Y. S. THAKARE
M.Sc. (CHE) Ph D, NET, SET
Assistant Professor in Chemistry,
Shri Shivaji Science College, Amravati
Email: yogitathakare_2007@rediffmail.com
B Sc- III Year
SEM-V
PAPER-III
PHYSICAL CHEMISTRY
UNIT- VI
Raman Spectroscopy
24-November -20 1

2. RAMAN
SPECTROSCOPY

3. CONTENTS
 Introduction
 About energy level diagram and study of
Microwave, IR spectroscopy.
 Need of studying Raman Spectroscopy.
 Principle
 Conditions for Raman active molecules
 Details about Raman effect
 Raman spectra and its characteristics
 Rotational Raman spectra
 Vibrational Raman spectra
 Applications
 References

4. INTRODUCTION
 Raman effect, an adjunct to electronic and infrared spectra
was predicted by Adolf Smekal (1923) and discovered by
C. V. Raman and his co-worker K. S. Krishnan in 1928.
 For his excellent works, C. V. Raman won Nobel Prize in
1930. Raman spectroscopic measurement is usually
carried out in the visible region(400-700 nm).
 Raman spectra relate to vibrational/rotational transitions in
molecules but in a different manner.
 In this case only scattering is measured but not absorption
of radiation.

5. Raman spectroscopy

6. About Energy Levels
 The branch of spectroscopy
can be studied under two
following heads
1) Atomic spectroscopy
2) Molecular spectroscopy
 In Molecular spectroscopy,
Electronic levels are
accompanied by number of
vibrational levels and similarly
vibrational levels are
accompanied by number of
rotational levels.

7. What is Microwave and IR Spectroscopy?
 The transition of electron from one rotational level to
another rotational level within same vibrational level is
known as Rotational Spectra. And such type of radiations
are available in microwave region hence rotational
spectroscopy is also known as Microwave
Spectroscopy.
 The transition of electron from one vibrational level to
another vibrational level within same electronic level is
known as Vibrational Spectra. And such type of radiations
are available in IR region hence Vibrational Spectroscopy
is also known as IR Spectroscopy.
 To obtain such spectra the molecule must be in gaseous
state and must posses permanent dipole moment.

8. Need of studying Raman Spectroscopy
 As we know that, molecule with no permanent diople
moment would have no pure rotational spectrum, and the
molecules which do not have vibrational oscillation would
have no IR absorption or emission spectra.
Raman spectroscopy allows us to determine
rotational and vibrational level spacing for such systems,
and hence to determine bond lengths and force constants
for such molecules. That is we can use Raman Spectrum
to study H2,O2,N2………etc

9. What happens when light falls on
a material?
Transmission
Reflection
Absorption
Luminescence
Elastic Scattering
Inelastic Scattering

10. Raman Spectroscopy
1 in 107 photons is scattered inelastically
Infrared
(absorption)
Raman
(scattering)
v” = 0
v” = 1
virtual
state
Excitation
Scattered
Rotational Raman
Vibrational Raman
Electronic Raman

11. Structure of CO2 molecule
The modes of vibrations of CO2 molecule are represented as
below:
The remaining three vibrations are IR active because that
produce changes in dipole moment of CO2 molecule.
The two bending vibrations have same energy, because they
are degenerate (doubly).
So, IR spectrum of CO2 molecule shows two absorption
bands.

12. Principle of Raman Effect
 When a
monochromatic beam
of light is allowed to
pass through a
substance in solids,
liquids or gaseous
state, the light is
scattered which is
perpendicular to the
incident radiation and
contain some
additional frequencies
over and above that of

13.  When a monochromatic beam of light was
allowed to pass through a substance in the
solid liquid or gaseous state the light is
scattered and contains some additional
frequencies over and above that of the
incident frequency. This phenomenon is
called as Raman Effect
 The lines having lower frequency that that of
incident frequency are called as stroke line
and those having higher frequency are called
as anti-stroke lines
 Line with the same frequency as the incident
light is called as Rayleigh lines.

Raman spectroscopy:

14. Experimental arrangement

16. If νi is the wave number (frequency) of incident radiation and νs that
of the radiation scattered by the given molecule, then the Raman
shift or Raman frequency (∆ν) is given by
∆ν = νi − νs
For stokes lines, ∆ν = +ve i.e. νi > νs or νs < νi
For Anti-stokes lines, ∆ν = −ve i.e. νi < νs or νs > νi
For Rayleigh lines ∆ν = 0 i.e. νi = νs
Thus, the Raman frequencies observed for a particular
substance are characteristic of that substance . The various
observation made by Raman are called Raman effect and the
spectrum observed is called as Raman spectrum. Raman
spectrum is shown below (Fig 1))

19. TRANSITION OF RAMAN AND
RAYLEIGH SCATTERING.
Elastic collision-
rayleigh scattering
(No change in energy)
Inelastic collision-
stokes scattering and
antistokes scattering.
(Exchange of energy)

20. Raman shift (∆ν) generally lies within the range falls in far and near
infrared region of the spectrum. This leads to conclusion that the
change in energy of the scattered light in Raman effect corresponds
to the energy changes accompanying rotational and vibrational
transition in the molecule.

21. Characteristics of Raman spectrum
The lines observed in Raman effect exhibit number of characteristics
which are given below. The intensity and width of Raman wing vary
from liquid to liquid and depends upon the anisotropy of the
molecule of the liquid
i) The intensity of stokes lines is always greater than the Anti-
stokes lines.
ii) Raman shift (∆ν) , generally lies within the far and near
infrared regions.
iii) Raman lines are symmetrically displaced about Rayleigh line.
iv) Raman lines are due to change in polarization of the
molecules.

22. Characteristics of Raman spectrum
 The intensity of stokes lines
is always greater than the
antistokes lines.
 Raman shift, Δν generally
lies within the far and near
infrared regions.
 Raman lines are
symmetrically displaced
about Rayleigh line.
 Raman lines are due to
change in polarization of the
molecules.
.
If νi is the frequency of incident radiation
and νs is of radiation scattered by the
given molecule, then the Raman shift is
given by,
 νi = νs Rayleigh
line
νi > νs Stokes
line
 νi < νs Anti
Stokes line
For stokes line: Δν =+ve i.e νi > νs
 For antistokes line: Δν = -ve i.e νi <
νs

23. Conditions for Raman active molecules
 For a molecule to be Raman Active, vibrations/
rotation should be accompanied by change in
polarization of the molecule.
 And that polarized molecules are effective at certain
wavelength which causes scattering of light.